1. Technical Field
The present invention generally relates to a radiofrequency communication unit, and in particular a radiofrequency communication unit for replacing a cable link between two electronic devices with a radio link when the distance is small between the two devices.
2. Discussion of the Related Art
Such a communication unit, of a range of a few meters, exchanges radiofrequency signals (having a frequency ranging between 1.8 and 10 GHz) by means of a small flat antenna, generally called in the art a patch antenna, coupled to a radiofrequency signal processing chip. Input/output pads of the unit enable the chip to exchange so-called “low frequency” signals (having a frequency ranging between 10 kHz and 10 MHz) with a device in which the unit is integrated.
FIG. 1 schematically shows a cross-sectional side view of a radiofrequency communication unit 2, comprised of a stratified substrate 4 formed of two dielectric substrates 6 and 8 arranged on either side of a conductive screen layer 10. A conductive layer 12 forming a patch antenna is printed on the upper surface of substrate 6. The lower surface of substrate 8 supports a printed radiofrequency antenna line 16 connected to a terminal 18 of a chip 20 intended to transmit or receive radiofrequency signals. Radiofrequency line 16 is coupled to antenna layer 12 by a coupling slot 22 made in the screen layer 10 perpendicularly to line 16. The lower surface of substrate 8 also supports printed tracks 24 which define a plurality of input/output pads (I/O) of the unit and their connection to terminals 26 (a single one of which is shown) of chip 20. Each of the input/output pads is formed of a metallized surface where a connection ball (or welding ball) is placed. At least one of the pads is provided to be connected to ground and at least another one is provided to be connected to a supply terminal of the unit; the other pads are provided to transmit low-frequency signals between chip 20 and the outside of the unit. At least one via 28 made in substrate 8 connects screen layer 10 to a grounded pad.
Coupling slot 22 is made in screen layer 10 vertically above a portion O of antenna line 16. Upon transmission, the radiation of portion O is captured by the antenna 12 which retransmits it. Upon reception, the unit operates symmetrically.
Such a unit operates satisfactorily, but a problem results from the fact that the welding balls arranged on the I/O pads, which enable a simple assembly with a low bulk, have a height limited to approximately 0.5 mm. This imposes assembling chip 20 head-to-tail directly on tracks 24 printed under substrate 8. Now, such an assembly imposes that the chip 20 and the substrate 8 have substantially identical thermal expansion coefficients to avoid occurrence of mechanical constraints likely to result in a tearing of the chip terminals. Thus, in the conventional case of a silicon chip 20, substrate 8 must preferably be made of glass. A glass substrate being very difficult to bore, the forming of via 28 requires great precautions. Further, glass is poorly wettable and the filling of via 28 with a conductive material is also difficult. All this substantially increases the unit manufacturing cost. It is, however, necessary for the voltage of the screen layer not to be left floating, since screen layer 10 captures the undesirable radiation of line 16 towards antenna 12 and the radiation of antenna 12 towards the inside of the unit. The voltage of screen layer 10, if it was left floating, would vary under the effect of the captured radiation and screen layer 10 would radiate in the radiofrequency field. Such a radiation would disturb the operation of antenna 12 and that of chip 20, which is not desirable.
A solution consists of replacing via 28 through substrate 8 by an external conductive track located on an edge of the substrate. However, the manufacturing of an external track remains difficult and expensive.